Optical fiber of the type used to carry optical signals is fabricated typically by heating and drawing a portion of an optical preform comprising a refractive core surrounded by a protective glass cladding. Presently, there are several known processes for fabricating preforms. The modified chemical vapor deposition (MCVD) process, which is described in U.S. Pat. No. 4,217,027 issued in the names of J. B. MacChesney et al. on Aug. 12, 1980 and assigned to Bell Laboratories, Inc., has been found most useful because the process enables large scale production of preforms which yield very low loss optical fiber.
During the fabrication of preforms by the MCVD process, precursor, reactant-containing gases, such as SiCl.sub.4 and GeCl.sub.4 are passed through a rotating substrate tube which is made of silica glass. A torch heats the tube from the outside as the precursor gases are passed therethrough, causing deposition of submicron-sized glass particles on the inside surface of the tube. The torch is moved along the longitudinal axis of the tube in a plurality of passes to build up layer upon layer of glass to provide a preform tube. Once a sufficient number of layers have been deposited, the preform tube is then heated to cause it to be collapsed to yield a preform or preform rod as it is often called.
Increased demand for optical fiber has prompted efforts to increase the productivity of the MCVD process. However, the MCVD process rate is limited by the thickness of the wall of the substrate tube. To obtain optical fiber having optimal optical and geometrical characteristics, the preform must have a core-to-cladding mass ratio within specified limits. Increasing the mass of the substrate tube to obtain a larger preform requires that the wall of the substrate tube be made thicker. Increasing the thickness of the wall of the substrate tube, however, reduces the rate of heat transfer to the reactant-containing gases, thereby increasing the time required to deposit each layer of glass particulates. If the wall of the substrate tube is too thick, then insufficient heat transfer may occur, which may result in the formation of bubbles or incomplete sintering.
One way in which the productivity of the MCVD process can be increased is first to produce an undercladded preform, having a larger than desired core-to-cladding mass ratio. This preform is inserted into a glass tube which is referred to as an overcladding tube and which is then collapsed onto the preform. This is referred to as the rod and tube technique. It is desirable that any added eccentricity of material about the preform core due to overcladding should be mimimized.
Insertion of the preform into the overcladding tube has been accomplished manually. Contact of the preform with the inside surface of the tube has not been found to be detrimental for present proof test levels of interest. However, radial misalignment between the overcladding tube and the undercladded preform should be minimized, otherwise the resultant drawn fiber core may be too eccentric which inhibits proper splicing of the drawn fiber to another. More sophistricated methods and apparatus for inserting a glass rod into a glass tube are known.
Collapse of the tube onto the preform rod while the tube and rod are mounted in a horizontal lathe has been acomplished using an oxy-hydrogen torch such as one shown in U.S. Pat. No. 4,231,777 which issued on Nov. 4, 1980 in the names of B. Lynch and F. P. Partus. Because that torch has a relatively wide hot zone which does not have a sharply defined end, it has been found that air pockets become trapped between the tube and the preform rod during collapse and manifest themselves as air lines in the fiber, resulting in fiber breaks at low proof test levels. This problem may be overcome by gaseous cooling of the tube ahead of the torch, but this increases the hydrogen demand in the torch and may increase the likelihood of contaminating the tube surface. Voids also may occur at the interface between the preform rod and the tube because of non-concentric collapse of the tube on the rod.
Instead of being accomplished on a horizontal lathe, the collapse of the tube on a preform rod has been acomplished in a furnace. Typically, this has been accomplished by inserting the preform rod into an overcladding tube and then moving the rod and tube coaxially through a draw furnace which causes collapse prior to the drawing of the fiber. See U.S. Pat. No. 4,547,644, which issued on Oct. 15, 1985 in the names of W. C. Bair et al for a typical optical fiber drawing furnace. The fiber drawing process itself is relatively unaffected by tube collapse during drawing. However, the optical fiber draw rate may be reduced if the time required for collapsing the overcladding tube is the rate-limiting step of the fiber drawing process. Further, centering of the tube and inserted preform rod may be a problem in using the furnace to collapse the tube onto the preform rod. However, this may be overcome by a centering technique referenced to the optical fiber instead of to the preform rod.
Collapse of an overcladding tube onto a preform rod on a lathe subjects the preform to an extra heat treatment which is not required for furnace collapse. However, by accomplishing collapse on a lathe, the heat treatment step also acts to provide fire polishing of the overclad preform. The fire polishing step which is a surface treatment that removes defects causes the proof test yield of the drawn fiber to be increased.
U.S. Pat. No. 4,505,729 issued to H. Matsumura et al. on Mar. 19, 1985, discloses a method for producing an optical fiber preform the steps of which are a variation of the rod and tube technique discussed above. According to the Matsumura et al. method, a layer of doped glass particles is deposited on the inner surface of a quartz substrate tube. A glass rod is inserted into the substrate tube. The substrate tube, with the rod coaxially inserted therein, is then collapsed while the internal pressure of the tube is reduced slightly to provide a preform in which a cladding or jacket thereof is elliptic in cross section.
What is needed and what seemingly is not provided by the prior art are methods and apparatus for overcladding expeditiously a preform rod without degradation of fiber strength or core concentricity. The sought after methods and apparatus that cause a glass tube, into which a glass preform rod has been inserted, to be collapsed onto the rod should result in a preform having substantially concentric layers with no air pockets at the interface between the rod and the tube. Desirably, such sought after methods and apparatus will increase the capacity of existing plant facilities. Inasmuch as increased fiber output can be more than offset by a decrease in the yield, care must be taken to insure that the sought after collapse process for rod and tube manufacture does not result in a decreased yield.